I Have a 1985 d100 with a slant six and a 3 speed overdrive manual tranny. The trouble I am having is a intermitent stumble. The truck runs fine when cold and after it warms up it begins to stumble. The truck is all original with 116000 miles on it. All vacuum lines are intact and connected. I have replaced the fuel filter, cap, rotor button, plugs, and the wires. The only things I have left are a fuel pump and the fuel lines themselves. The air filter appears fresh and I have also drained the tank and refilled with fresh fuel. Timing is correct with the sticker under the hood and my factory service manual. Any help with this particular issue would be appreciated.

Two things to check:

1) With the engine off and the choke open, remove the air cleaner lid. Operate the throttle with your hand as you peer down into the carburetor. You should see a good strong shot of gasoline squirted into the middle of the carburetor every time you operate the throttle. If it's a weak dribble, or there's none at all, your accelerator pump needs attention.

2) Check the heat riser valve. This is the ~3" round disc facing forward, located in the central collector area of the exhaust manifold. With the engine cool (otherwise you'll burn yourself) grasp the disc and try to rotate it. It should rotate freely counterclockwise a little more than half a turn, then when you let go, it should spring back to its clockwise position. If that's not the case, your manifold heat control valve needs attention.

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